The Prius, like the Honda Civic Hybrid and the forthcoming Insight, is a parallel hybrid that uses both an electric motor and a gasoline engine to drive the wheels. It is designed to deliver optimal fuel economy at low speed or in stop-and-go traffic, when the electric motor does all the work. At highway speeds, it’s just another fossil-fuel burner, albeit one that gets 45 mpg and emits less CO2 than almost anything else on the road.

The Volt, which General Motors finally unveiled Tuesday, is a series hybrid, also called a range-extended electric vehicle. Like the Prius, it’s got an electric motor and a gasoline engine, but the engine merely charges the battery as it approaches depletion. Electricity alone turns the 17-inch wheels. The Volt is designed to travel 40 miles on a single charge of its lithium-ion battery, meaning most drivers will never burn a drop of gasoline.

Assuming it works, of course.

GM is confident it will, and it’s given 700 people — many of them veterans of the groundbreaking EV1 electric car GM unceremoniously killed in 1994 2003 — a blank check to make sure the Volt is in showrooms by the end of 2010. The company reportedly will spend $400 to $500 million on the project during the next two years. "We can do anything we want to make this happen," Andrew Farah, the Volt’s chief engineer and a veteran of the EV1, tells us. Many industry analysts and battery experts say it’ll be close, but GM almost certainly will meet that deadline.

"GM is staking its reputation on the Volt working and it’s spent a lot of money to make sure it will work," says Mike Omotoso of JD Power & Associates. "I think they’ll be able to mass produce them by 2010."

The heart of the car is a T-shaped 16-kilowatt-hour battery comprised of 220 lithium-ion cells and a 111-kilowatt (150-horsepower) electric motor good for a top speed of 100 mph. GM says the drivetrain will produce acceleration similar to that of a V-6 engine. The goal is to get the battery down to 396 pounds and no more than 64-inches long and 33 1/2-inches wide across the top of the "T." That’s light-years ahead of the similarly shaped lead-acid battery that powered the earliest EV1s; it weighed 1,200 pounds and was 92.5-inches long. The Volt’s battery will run the length of the cabin, taking up the space beneath the center console and the rear seat.

GM is testing batteries around-the-clock at labs in Michigan and Detroit, where engineers have as many as 40 battery packs on test rigs that measure life-cycle depletion rates, thermal behavior and load performance. "Extreme cold temperature and battery life are the biggest challenges," Denise Gray, director of advanced battery technology, says. The objective is to build a battery that works as well in Nome, Alaska or Flagstaff, Arizona as it does in the lab — and is good for 150,000 miles. "It’s a high hurdle to clear," Gray concedes. "Maximum" Bob Lutz, VP of global development for GM and the guy cracking the whip to keep the Volt on schedule, says the batteries are performing
"flawlessly" and "it’s almost scary that we aren’t seeing any problems with them." GM is testing batteries from LG Chem/Compact Power and A123 Systems/Continental, and Lutz says the company’s decided who’ll get the contract but won’t announce it until the end of the year.

General Motors wants the Volt to recharge in eight hours using a standard 120-volt wall outlet or three hours with a 240. Of course, that won’t do you any good if you’re miles from home when the batteries are winding down. At that point, the Volt’s 1.4-liter four-cylinder engine kicks on, powering a 53-kilowatt generator that will keep the battery going. The original plan called for a 1-liter three-cylinder turbocharged engine, but GM went with the four because it’s lighter and simpler. "To be honest with you, we’ve got enough technology in the Volt," says Micky Bly, director of hybrid drivetrain engineering. "We don’t need the added complexity of a turbocharger."

Bly says the engine will produce less than 100 kilowatts (134 horsepower) but promises that’s enough to do the job. And because the engine drives a generator that will run at a constant speed, the power band can be optimized for maximum fuel efficiency and lowest emissions. "We can run it in the sweet spot at all times," he says. Just how sweet that spot is remains to be seen, because GM isn’t saying what kind of fuel economy or emissions we’ll see from the Volt, although 50 mpg has been mentioned.

The engine will not fully charge the battery. Instead, it will keep the battery in what Farah calls "charge sustaining mode" at about 30 percent of its capacity, providing enough juice to keep the car going. The idea, like so much of the technology in the Volt, was born of the EV1. Engineers testing the EV1 in the early 1990s needed a way to keep its battery charged as they racked up miles on the track. They fashioned a generator from a snowmobile engine strapped to a trailer towed behind the car. Farah thought it was a great way to improve the EV1’s range, and some of the engineers urged GM to incorporate it into the car.